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Income Taxes (Narrative) (Detail) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Effective Income Tax Computation [Line Items] |  |  |
Federal statutory income tax rate | 21.00% | 35.00% | 35.00%
Percentage of deductibility of interest expense | 30.00% |  |
Percentage of federal net operating loss carryforwards | 21.00% |  |
Income tax reconciliation increase in deferred income tax expense | $ 23 |  |
Dominion Energy Gas Holdings, LLC |  |  |
Effective Income Tax Computation [Line Items] |  |  |
Federal statutory income tax rate | 21.00% | 35.00% |
Increase (decrease) in regulatory liabilities | $ 35 |  |
Increase in income tax expense returned to customers | 8 |  |
Increase in regulatory liabilities returned to customers | $ 11 |  |

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